These maps were produced by the U.S. Geological Survey (USGS) as part of an effort to map the entire United States. The 1901 map was compiled using data obtained entirely by field crews. It was lithographed in three colors from hand-engraved copper plates, a method that could not effectively produce large areas of solid color. Thus, Lake Michigan is represented by “water lining,” the technique of drawing increasingly simplified replications of the shoreline inward toward the center of the water body.
Mapping and printing techniques had changed significantly by 1958, when the second map was printed. The map was compiled stereoscopically from aerial photographs—a technique scarcely dreamed of in 1901. This map uses five colors and represents Lake Michigan with blue tint. Depth curve lines and sounding values have been added to Lake Michigan from U.S. Lake Survey charts.
These are known as 15-minute topographic “quadrangle” maps, because they are four-sided and cover 15 minutes of latitude and 15 minutes of longitude. These particular quadrangles are bounded by the following parallels and meridians: